OnSolar Blog

Your Astronomical Society kindly prepared the following article to tell us a little about their 12V DC solar PV powered lighting system.

P Buglass says: Since we officially opened the YAS Observatory at our “secret location”, we had plans to provide electric lighting, both white and red.  The provision of suitable lighting was made more awkward by the fact that there is no mains electricity available at our site, therefore any power we use would have to be either brought with us, or generated at the site…

Our previous observatory took the first approach and we used a 12V car battery which had to be periodically taken away to be recharged.  This was far from ideal and we never knew quite when the battery would run out leaving us in the dark, literally.

For the new observatory we decided on a different approach and settled on a design which incorporated a solar voltaic panel and a number of deep cycle 12V batteries.  The power system was installed last Summer and has been tweaked over the Winter and is now able to provide all the lighting needs of the observatory.

The set-up consists of a 43W solar panel which uses a 4 Amp charge controller to change a pair of 12V 115AHr deep cycle batteries, wired in parallel to provide 230AHr of capacity.  A more expensive mono-crystalline solar panel was selected due to its smaller size/power ratio as it had to fit inside our cabin windows. 

View of the 43w PV panel as seen from the inside

Two 12v 110Ah Lead Acid 'Gel' batteries in parallel complete the battery store

The regulator ensures a stable charge rate to the battery set

Our original plan was to mount the solar panel on the roof of our portacabin, but we were concerned about security and didn’t want to make it too visible.  Some maths was performed to look at the angle of the Sun in Winter and Summer, and we decided to mount our panel vertically in one of the cabin’s south facing windows.  The maths showed that in Winter when the Sun was low, we would actually generate more power than if the panel had been roof mounted at 18 degrees (an angle we had decided was the maximum we could use and still keep the panel invisible).  Therefore we decided to go for window mounting to maximise our Winter power, (the time of year we need it the most).  In Summer time, the Sun is much higher, and therefore the vertical panel is much less efficient, but the overall power generated due to the longer days makes up for this.

Under bright sunlight the panel can put almost 3Ahr into the battery bank, but a realistic figure is closer to 2Ahr in Winter time.  Even with a conservative estimate of only 1 hour per day of bright sunlight during the winter months, we can still expect to put over 12Ahr into the batteries during the week, which will allow us to use our lights for 6 to 8 hours at a weekend without depleting the batteries on a week by week basis.  In Spring and Autumn the figures are more like 30Ahr a week into the batteries, which is more than enough for a few nights of observing at the weekends.  In the Summer time we will have power to spare so we will be holding lots of summer BBQs and other evening activities . . . . Bingo anyone?

As with any 12V power system, protective fuses and thick cables are essential.  Power loss in a cable is proportional to the square of the current, so with a low voltage system more current is required for a given power, thus increasing the wiring losses significantly.  Power loss = I²R, where I is the current drawn, and R the resistance of the wires, so reducing wiring resistance by using thick cables reduces power loss.  We used the thickest mains cable we could lay our hands on, and it seems to work well with minimal power loss indicated by the voltage level hardly dropping under load.  Where we have a long cable run down the inside of our 40 ft steel shipping container observatory area, we have used a double run of thick twin and earth cable, with all the separate conductors, wired together giving one cable for positive and the other for negative.  This may be overkill, but we can pull quite a few Amps through this wiring arrangement with minimal voltage drop at the far end.

The power feed out of the batteries is protected by a 30A in-line fuse (car spade type for easy replacement), and this will likely be up-rated to 40A, or more, in the future as additional power hungry equipment is installed in the observatory such as a mains AC voltage inverter to power laptops, etc. 

The white lights we use are low energy, compact fluorescents, rated at 7W and 11W, and take less than 1A from our system, but they are unique in that they are designed to operate directly off a 12V DC supply, so they contain an integrated 12V inverter to power them.  They look just like other low energy light bulbs you would find in your house, and give a similar light level you would expect from your bulbs at home.  The use of ES (screw) fittings is desirable, as these are easier to ensure the correct polarity is maintained, however the bulbs can be bought in either ES or Bayonet fittings.

Low energy 12V CFL lighting

The warm room cabin has been provided with a single 11W bulb, with a switch beside the cabin door, which provides adequate light for most activities such as reading, committee meetings, etc.  A 7W bulb has been installed in a bulkhead fitting on the observing deck for setting up and clearing away purposes, again with it’s own adjacent switch.  Finally there is another 11W bulb and switch above the main door into the container, but again the use of this light will be limited to setting up and clearing away, as both this light and the observing deck light will normally be turned off during observing, when red lights will be used exclusively.

The red light in the observatory is provided by a number of small 12V festoon lights, which contain 8 red LEDS each.  One red light is located next to the container door with a master red light switch, and two more red lights illuminate the stairs up to the observing deck.  On the observing deck itself there are 2 additional red lights, but these can be switched on and off independently of the other red lights if needed, i.e. if CCD imaging was being undertaken and all lighting on the observing deck needed to be extinguished.

Now that the observatory lighting is fully operational, we are working on installing a table area for laptop PCs in the cabin, which will have a small mains power inverter, running off our 12V supply.  This will enable observers to use and recharge their laptops as needed.  We will also be able to tap off  12V feeds to the telescope pillar, and the 12” other Newtonian telescope drive system, in the latter case again utilising a small mains inverter.

If the additional power demands of the laptop station and telescope drives prove to be taking more out of the batteries than we can put back during the week, we have the option of installing a second 43W solar panel in another south facing window, thus doubling our power generation capacity.

It has been a very rewarding exercise to equip the YAS observatory with a “renewable” energy lighting and power system, and it has demonstrated that something similar could be done on a domestic scale to provide limited household lighting and power too.  Holding committee meetings in the observatory with a bright light available at the flick of a switch, without having to fire up the petrol generator, is a great convenience, and one which shouldn’t be underestimated.

The costs of the major system components were £249 for the solar panel, about £50 for each battery, and approximately £10 per 12V low energy bulb.  The wiring and other ancillary switches, etc, have come mainly from our “bits and bobs” boxes.  Thanks should go out to all those who have helped me put the power system together and get it wired up and working.

I feel it is very appropriate and fitting, for us to be utilising power derived directly from our nearest star, the Sun, to help us in our night time observations of other stars and celestial objects.  Long live “Star Power”!

P Buglass, York Astronomical Society,  April 2009. http://www.yorkastro.org.uk/

We would like to say Thank You to P Buglass for telling us about his solar PV project and hope that he continues to enjoy the benefits of the ‘Off Grid’ 12v solar PV power system. Long may your 12v cfl lamps perform! Ken. OnSolar.co.uk